January 29, 2012

UCLA researchers have explained the puzzling disappearing act of energetic electrons in Earth's outer radiation belt, using data collected from a fleet of orbiting spacecraft.

In a paper published today in the journal Nature Physics, the team shows that the missing electrons are swept away from the planet by a tide of solar wind particles during periods of heightened solar activity.

"This is an important milestone in understanding Earth's space environment," said lead study author Drew Turner, an assistant researcher in the UCLA Department of Earth and Space Sciences and a member of UCLA's Institute for Geophysics and Planetary Physics (IGPP). "We are one step closer towards understanding and predicting space weather phenomena."

During powerful solar events such as coronal mass ejections, parts of the magnetized outer layers of sun's atmosphere crash onto Earth's magnetic field, triggering geomagnetic storms capable of damaging the electronics of orbiting spacecraft. These cosmic squalls have a peculiar effect on Earth's outer radiation belt, a doughnut-shaped region of space filled with electrons so energetic that they move at nearly the speed of light.

"During the onset of a geomagnetic storm, nearly all the electrons trapped within the radiation belt vanish, only to come back with a vengeance a few hours later," said Vassilis Angelopoulos, a UCLA professor of Earth and space sciences and IGPP researcher.

The missing electrons surprised scientists when the trend was first measured in the 1960s by instruments onboard the earliest spacecraft sent into orbit, said study co-author Yuri Shprits, a research geophysicist with the IGPP and the departments of Earth and space sciences, and atmospheric and oceanic sciences.

"It's a puzzling effect," he said. "Oceans on Earth do not suddenly lose most of their water, yet radiation belts filled with electrons can be rapidly depopulated."

Even stranger, the electrons go missing during the peak of a geomagnetic storm, a time when one might expect the radiation belt to be filled with energetic particles because of the extreme bombardment by the solar wind.

Where do the electrons go? This question has remained unresolved since the early 1960s. Some believed the electrons were lost to Earth's atmosphere, while others hypothesized that the electrons were not permanently lost at all but merely temporarily drained of energy so that they appeared absent.

"Our study in 2006 suggested that electrons may be, in fact, lost to the interplanetary medium and decelerated by moving outwards," Shprits said. "However, until recently, there was no definitive proof for this theory."

To resolve the mystery, Turner and his team used data from three networks of orbiting spacecraft positioned at different distances from Earth to catch the escaping electrons in the act. The data show that while a small amount of the missing energetic electrons did fall into the atmosphere, the vast majority were pushed away from the planet, stripped away from the radiation belt by the onslaught of solar wind particles during the heightened solar activity that generated the magnetic storm itself.

A greater understanding of Earth's radiation belts is vital for protecting the satellites we rely on for global positioning, communications and weather monitoring, Turner said. Earth's outer radiation belt is a harsh radiation environment for spacecraft and astronauts; the high-energy electrons can penetrate a spacecraft's shielding and wreak havoc on its delicate electronics. Geomagnetic storms triggered when the oncoming particles smash into Earth's magnetosphere can cause partial or total spacecraft failure.

"While most satellites are designed with some level of radiation protection in mind, spacecraft engineers must rely on approximations and statistics because they lack the data needed to model and predict the behavior of high-energy electrons in the outer radiation belt," Turner said.

During the 2003 "Halloween Storm," more than 30 satellites reported malfunctions, and one was a total loss, said Angelopoulos, a co-author of the current research. As the solar maximum approaches in 2013, marking the sun's peak activity over a roughly 11-year cycle, geomagnetic storms may occur as often as several times per month.

"High-energy electrons can cut down the lifetime of a spacecraft significantly," Turner said. "Satellites that spend a prolonged period within the active radiation belt might stop functioning years early."

While a mechanized spacecraft might include multiple redundant circuits to reduce the risk of total failure during a solar event, human explorers in orbit do not have the same luxury. High-energy electrons can punch through astronauts' spacesuits and pose serious health risks, Turner said.

"As a society, we've become incredibly dependent on space-based technology," he said. "Understanding this population of energetic electrons and their extreme variations will help create more accurate models to predict the effect of geomagnetic storms on the radiation belts."

Key observational data used in this study was collected by a network of NASA spacecraft known as THEMIS (Time History of Events and Macroscale Interactions during Substorms); Angelopoulos is the principal investigator of the THEMIS mission. Additional information was obtained from two groups of weather satellites called POES (Polar Operational Environmental Satellite) and GOES (Geostationary Operational Environmental Satellite).

A new collaboration between UCLA and Russia's Moscow State University promises to paint an even clearer picture of these vanishing electrons. Slated for launch in the spring of 2012, the Lomonosov spacecraft will fly in low Earth orbit to measure highly energetic particles with unprecedented accuracy, said Shprits, the principal investigator of the project. Several key instruments for the mission are being developed and assembled at UCLA.

Earth's radiation belts were discovered in 1958 by Explorer I, the first U.S. satellite that traveled to space.

"What we are studying was the first discovery of the space age," Shprits said. "People realized that launches of spacecraft didn't only make the news, they could also make scientific discoveries that were completely unexpected."

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14 comments

Further proof that space exploration dollars is money well spent. Science needs to be free to answer all of mankind's questions about everything, everywhere. That means the whole Universe and any/all mini-universes contained in it. All dimensions, even imaginary ones envisioned by many String Theory enthusiasts. Dark Matter and Energy, to Earth's Climate and ways to understand and combat Global Warming. We need to keep reaching for the stars to survive when Earth stops being habitable in a million years or so, or much sooner, if we fail to be good stewards of our home. It IS a matter of Humanity's survival in the long run. Plus its just so cool.

Re: "The data show that while a small amount of the missing energetic electrons did fall into the atmosphere, the vast majority were pushed away from the planet, stripped away from the radiation belt by the onslaught of solar wind particles during the heightened solar activity that generated the magnetic storm itself."

Guys, language is *incredibly* important to conceptual meaning. Do electrons "fall"? Can I fill a glass with a cup of these "falling" electrons? Is lightning what happens when electrons "fall" from the sky?

When we consider the enduring mysteries of science, should we not also wonder if the mystery results from the language which we use? And from where does this sloppy language originate? Is it not also possible that a gravity-based framework is simply incapable of explaining large-scale electric cosmic currents? Maybe the real conversation we should be having pertains to why moving electrons are not called "electric currents" ...

q/"During the onset of a geomagnetic storm, nearly all the electrons trapped within the radiation belt vanish, only to come back with a vengeance a few hours later," said Vassilis Angelopoulos, a UCLA professor of Earth and space sciences and IGPP researcher./q

Solving the mechanics of this "comeback" is the key to themystery. I suspect a circuita large scale electric circuit or loop is created under these conditions with a periodicity based on the magnitude of the storm's energy. Analogous to the pulse of DC current flowing through an electric fence.

Perhaps then, the mechanism is an indirect one: the solar wind particles strike the more massive, positively charged cation species which then rebound off of positively charged cation species, sending some towards, but most away from, the earth, and loosing kinetic energy in the process (maybe allowing some recombination); The far lighter, negatively charged electrons then follow the cations away from or towards the earth.

Electrons and protons have different masses. While the sumtotal of solar wind is electrically neutral if you accelerated protons and electrons by the same magnetic event (i.e. they experience the same field strength) they will be accelerated to different speeds.Though the resultant field strength between the proton front and the electron front should eventually compensate that. Probably even overcompensate so that for some time you get an oscillating charge distribution in the wavefront.

Maybe we're just seeing an effect of electrons being either caught in the 'proton front' of the wave or displaced by the 'electron front' of the wave.

So, is there a way to figure out whether the electrons are combining with protons from the solar wind and leaving as neutral hydrogen? Should we be looking for sign of increased neutral hydrogen abundances during these events? Just thinking out loud...

...nearly all the electrons trapped within the radiation belt vanish, only to come back with a vengeance a few hours later...

How can we be sure they're really "lost to the interplanetary medium and decelerated by moving outwards", as they say the data indicates, when the "coming back with a vengeance" part implies to me that the topography of the electric curr-, excuse me, radiation belt was altered and then reestablished, or simply returned to its basic state after the impinging force was removed, or (depending on when it returns) if it simply catches up and compensates if it returns to its norm while the storm continues. Unless I missed it the article wasn't all that clear as to whether the "a few hours later" was before or after the storm had ended...